The present invention relates to the field of coating technology, in particular a primer coating for the use with polymeric coating systems for materials including but not limited to wood, metals, rubber, thermoplastics, paper and textiles, where the primer retains reactivity for a period of time after a first curing process is completed.
State of the art processes for fabricating multilayer coatings use actinic radiation in combination with photocurable prepolymer compositions that contain reactive monomers and oligomers, fillers and additives such as leveling agents to rapidly achieve highly cross linked polymer coatings. Typically, multilayer coatings are composed of thin layers that are applied in the wet state and cured sequentially. To ensure good adhesion between the wood or metal and the multilayer coating, a primer is usually employed. In the case of wood, this primer is sometimes waterborne to ensure good adhesion to the wood, and for metal, the primer layer is often a silane compound, where one end of the molecule covalently bonds to the metal substrate and the other end of the molecule covalently links with the first coatings layer.
To ensure adhesion between layers, state of the art processes utilize a controlled level of oxygen inhibition to ensure that reactive compounds remain to covalently bond to the next applied layer upon additional cure.
In the art, off stoichiometry has long been well known and one patent describing thiol-ene polymers (U.S. Pat. No. 3,697,396) claims significantly off-stoichiometric mixtures, 0.5/1 to 2/1 ene to thiol ratio, and gives examples of materials fabricated via significant off stoichiometry and reports curing time and shore hardness of said materials. Single shaped molded cast articles from thiol-enes are claimed, while micropatterning and assembly of articles from separately fabricated thiol-ene pieces are not mentioned.
Off stoichiometric formulations have been previously described in the art. In a work by Khire et al. (Adv. Mater. 2008, 20, 3308-3313), very thin nanopatterned off stochiometry thiol-ene films were fabricated using a nanopatterned PDMS stamp. The prepolymer contained a small excess of thiols and the thiol groups present on the polymer surface after the polymerization was utilized for subsequent surface modification via a grafting-to process. Off stoichiometry was also used to control the thickness of the grafted layer where, by adjusting the thiol to ene ratio, oligomers of a predetermined average size where polymerized in bulk and attached to the thiol excess polymer surface. While off stoichiometric formulations have been known in the art and sometimes are used, no systematic investigation into the properties of off-stoichiometric formulations have been performed. On the contrary, it is often argued that off-stoichiometry results in very poor mechanical properties, and should be avoided, (Belfield et al. ACS symposium series 2003, p 65). The reasons for this are twofold: firstly deviation from stoichiometry results in a non-optimized polymeric network with less than the maximum number of crosslinks and the inclusion of dangling chain ends; and secondly, there is a finite risk that monomers are left unreacted in the network, thus risking leaching into the environment.
Ternary prepolymer formulations have been previously described in the art. Carioscia et al. (J. A. Carioscia et al., Polymer 48, (2007) 1526-1532) described the cure kinetics and Tg of a ternary prepolymer formulation consisting of a thiol an allyl and an epoxide monomer. In the mixture there was also added a radical initiator and anionic initiator. While good ultimate mechanical properties were achieved, no attempt to temporally separate the dual cure events to utilize the inherent reactivity after an initial cure was attempted nor was such a strategy suggested.
U.S. Pat. No. 5,821,305 discloses a cross linked epoxy resin, where the epoxy has been cross linked with a carboxylic anhydride, where the anhydride is a copolymer of an ethylenically unsaturated anhydride and a vinyl compound. Triallylcyanurate is present.
U.S. Pat. No. 4,755,571 discloses a curable composition comprising an acrylic ester, an epoxide resin and a curing agent which can be a polymercaptan. The compositions can be used as adhesives and for fixing surfaces together.
For micropatterning of polymers, commercial as well as in-house thiol-ene formulations using both molding and direct photolithography have been described in the art. In (D. Bartolo, et al, Lab Chip, 2008, 8, 274) a method utilizing NOA 81, a commercially available thiol-ene based UV-curable glue, was shown to result in microfluidic devices, with adequate mechanical and bulk materials properties. Also good bonding to a substrate was shown upon renewed polymerization of an oxygen inhibited uncured polymer layer situated on the bottom of the device. Furthermore, it was claimed that oxygen inhibition, due to the high gas permeability of the PDMS mold, was effective for creating a layer of unreacted prepolymer on the channel surfaces, which is useful for subsequent surface modifications. In another example (J. Ashley et al. Lab Chip, 2011, 11, 2772-2778), a freeform UV-curable photolithographic technology using thiol-enes was shown. Uniquely, the propensity for unwanted cure in shadow regions exhibitied by thiol-enes, due to the high mobility of radicals and low propensity for inhibition, was hindered by a large amount of inhibitor added to the mixture.
WO 2012/042059 discloses a method of manufacturing articles comprising reacting a compound comprising at least two thiol groups and a compound comprising at least two carbon-carbon double bonds in off stochiometry ratios to obtain an intermediate article. That intermediate article comprises at least one unreacted group selected from an unreacted thiol group and an unreacted carbon-carbon double bond, and is subsequently contacted with a second article, wherein the surface of the second article comprises reactive groups. Subsequently the unreacted groups on the intermediate article are reacted with chemical groups on the second article to obtain covalent bonds and forming a final article. The first and/or the second article may also comprise at least one epoxide group.